Thin-film substrates or commonly also called flexible substrates are flexible sheet substrates that are usually made of a plane organic material and provide plane-applied electrical conductor structures onto which often electrical components are placed. Flexible substrates processed in this manner, with a substrate thickness of just approximately 10 μm, find a variety of applications in a suited manner for connecting electronic components electrically or mechanically. Production of these electrically and mechanically conductive connections usually occurs using a method generally referred to as “bonding”. For the technique of bonding, see printed publication DE 198 31 876 A1 to which, moreover, reference is regarding the terminology used herein. Furthermore, DE 37 03 694 A1 gives exact details regarding the so-called ball bonding method. Also dealing with herewith is DE 414 31 4123 A1, which describes a ball-based method of bonding for semiconductor chips and the subject matter of which is the production of tower-like connecting structures.
In addition to connecting microelectronic components on flexible substrates processed with conductor structures, two or a plurality of flexible substrates are often connected to each other, particularly in the field of mounting and connecting techniques (AVT) to form highly complex microsystems. According to the present state of the art, this occurs by means of gluing, soldering, welding or thermocompression bonding.
When gluing at least two flexible substrates, a conductive adhesive in the form of electrically conductive polymers, that are applied to certain locations on the to-be-connected flexible substrates, is used. When contacting the to-be-connected flexible substrates, there is, however due to the capillary forces between the substrates, only a difficult to control flow of the viscous adhesive. If nonetheless, flexible substrates with filigree-designed conductor structures are to be connected using gluing techniques, it is necessary to resort to complicated measures, which are inevitably accompanied by high costs.
On the other hand, joining two flexible substrates using prior art soldering techniques only permits producing an electrical connection, because metallizations applied in the form of electrical conductor structures must be present on the surfaces of the flexible substrates in order to produce soldering connections between two flexible substrates. Thus, it is impossible to join flexible substrates with electrical conductor substrates made of conductive polymers using the as such prior art soldering methods. Moreover, as in medicine, only biocompatible material can be employed, soldering is unsuited due to the soldering materials and their process products.
With the prior art welding techniques, it is presently impossible to produce both a mechanical as well as an electrical contact in a single process. Thus, utilizing the usual materials to connect two flexible substrates requires at least two different joining steps, in which, on the one hand, plastic welding and, on the other hand, metal welding is employed. Metallization welding requires that the conductive layer has a certain minimum thickness, which usually is a multiplicity of the thickness of a thin-film substrate metallization such as generated by sputtering or vapor depositing.
In this context, DE 35 12 237 A 1 discloses a connecting method for producing a multi-layer flexible circuit arrangement in which two adjacent circuit carriers are joined by means of ultrasonic welding. With regard to the ultrasonic welding method of joining, the aforecited drawbacks need to be mentioned.
Finally, similar to the soldering methods, the thermocompression bonding method requires a metallization layer, thus making production of a solely mechanical connection of flexible substrates, which usually are made of organic materials, by means of thermocompression bonding impossible.